LCDs describe a broad class of display devices that are used in a variety of applications. Some of the applications are directed to information displays and range from low to high information content. Some examples of low information content displays are watches, calculators, gas pump counters, and portable video games. Some examples of high information content displays are laptop computers, flat panel television screens, video projectors, and head mounted virtual displays.
Although each specific LCD product or application will impose specific constrains on the design, fabrication, and operation of the device, there are fundamental structural components, which are shared by almost all LCD devices. FIG. 1 shows a cross-sectional view of the basic components of a LCD device. LCD device 10 is comprised of liquid crystal (LC) material 7, which is contained, from below, by lower alignment layer 3, which is in contact with lower electrode 2, which is in further contact with lower substrate 1. From above, upper substrate 4 is in contact with upper electrode 5, which contacts upper alignment layer 6. Upper alignment layer 6 is in contact with LC material 7. In the case of high information content displays, the components of LCD device 10 shown in FIG. 1 represent the basic components of a typical single pixel. A typical display is comprised of a plurality of such pixels arranged in either an array or other geometric fashion. The geometry of some of the components, such as the electrodes, may depend on the display of interest, the exact configuration being selected by the device designer. In the following discussions, descriptions of the structure of a LCD device as in FIG. 1 and subsequent figures will be used to described either a display or a pixel interchangeably. When an assembly or device is described in the following sections, it will be understood by one skilled in the art that the device, assembly, layers, and electrodes can be either individual pixels, an entire area of pixels, or the entire display area.
Liquid crystals are noted for their anisotropic electrical and optical properties. The optical anisotropy causes birefringence due to the molecular structure and orientation of the LC material. It is known, to those of skill in the art, that the birefringence caused by the optical anisotropy of the LC molecules affects the state of polarization of a polarized light wave passing through the LC layer. Additionally, it is known to those of skill in the art, that when an electric field is applied between the electrodes and across the LC layer, the LC molecules align themselves with respect to the field, the specific orientation depending on the magnitude of the dielectric anisotropy.
Applying a voltage across the LC layer, using the upper and lower electrodes, operates a LCD device. The electrical anisotropy of the LCD material causes a deformation of the LCD material from its equilibrium position, in response to the applied voltage, and can induce the desired modulation of the polarized light passing through the LC layer in cooperation with a suitable analyzer. The viewer of the device will see a change in the intensity of the observable light. The degree of modulation and hence the intensity of the light observed by the viewer is typically determined by the amplitude of the voltage placed across the LC layer. For proper operation of a LCD device it is necessary to apply any deforming voltage as a purely AC signal, with no net DC voltage being placed across the LC layer. Neither should there be any intrinsic DC offset existing within the LCD device due to the properties of the device.
Undesirable effects caused by the DC fields are observable to the viewer in the form of flicker, image sticking, and voltage shielding. FIG. 2 shows some of the characteristics of a LCD device with and without an intrinsic DC offset. Characteristics of a prior art LCD device containing a DC offset are shown in 2b. The corresponding characteristics of a LCD device, according to the present invention, are shown in 2a. If a DC voltage is placed across the electrodes of a LCD device or if a LCD cell has an intrinsic, DC offset potential, differences in the observed intensity for the positive and negative polarities of the applied AC waveform will appear. This results in the two curves for observed intensity 15 instead of the single curve for observed intensity 12 which would exist if a DC offset was not present. An asymmetry of the observed light is shown in temporal transmission 16, which will be detected by an observer in the form of undesirable flicker.
In the previous discussion, description of the effect of DC offset is typical of a LC display utilizing a nematic LC material. There are many other types of LC materials, i.e., ferroelectrics, cholesterics, for which the presense of a DC offset may have similar or different effects on the observed intensity.
It is often necessary to correct for the presence of the DC offset by making changes in the fabrication of the LCD device, or by altering the drive waveform. However, the most common way to measure the DC offset is with the finished LCD device. Although this prior art method allows for a statistical determination of the distribution of DC offset for a sample of manufactured devices, there has not been a known way to measure parameters of the LCD device during fabrication that can be used to assess the DC offset that can be expected when the LCD device is completed. To solve this fundamental problem, methods and apparatus for predicting the intrinsic DC offset in a LCD device were disclosed in co-pending, commonly assigned U.S. patent application Ser. No. 09/863,212 entitled “Method and Apparatus For Predicting DC Offset Potential In a Liquid Crystal Display (LCD) Device,” filed concurrently with this application.
A typical prior art technique for minimizing DC offset, commonly used in transmissive displays, is to fabricate both the upper and lower assemblies equivalently. Typically, Indium-Tin-Oxide (ITO) is used for the electrodes and the same alignment layers are fabricated on both the upper and lower substrates. This technique has been found to produce very low or zero DC offset displays. In the case of reflective displays, it is difficult to manufacture the upper and lower assemblies equivalently by using identical materials for the electrodes and corresponding layers in between the electrodes and LC material.
Prior art attempts at solving the problem of intrinsic DC offset in reflective type LCD devices have not been effective. A prior art attempt at minimizing the DC offset potential in a reflective type of LCD device is U.S. Pat. No. 5,764,324, Lu et al., “Flicker-Free Reflective Liquid Crystal Cell,” [Lu] teaches the minimization of intrinsic DC offset potential by coating the reflective electrode on the lower substrate with a layer of ITO, which is the same electrode material used for the upper electrode in the LCD device. The purpose is to minimize the difference in work function between the two electrodes. Lu's method has two key limitations. The first is that the DC potential that will exist in the finished LCD device is not dependent solely on the work function of the two electrodes, but rather on the surface potential of the combined layers that are used in the fabrication of the LCD assembly. This is especially true if the cell construction above the ITO layer is not symmetric. For example there may be a passivation layer, such as SiO2, on top of one of the ITO layers to prevent against short circuits between the top and bottom electrodes. Secondly, in many specific LCD devices, coating the reflector layer with ITO is not technically feasible due to inter-pixel shorts, or is cost prohibitive. What is lacking in the art are methods of adjusting the difference in the surface potential between the lower and upper assemblies of an LCD device and LCD devices that have reduced or zero DC offset potential, especially for nonsymmetrical LCD devices which are required in reflective LCD designs.